Ansa-metallocene
Encyclopedia
An ansa-metallocene is a type of organometallic compound containing two cyclopentadienyl
ligand
s that are linked by a bridging group such that both cyclopentadienyl groups are bound to the same metal. The link prevents rotation of the cyclopentadienyl ligand and often modifies the structure and reactivity of the metal center. Some ansa-metallocenes are active in Ziegler-Natta catalysis, although none are used commercially. The term ansa-metallocene (ansa being Greek for "handle") was coined by Lüttringhaus and Kullick to describe alkylidene-bridged ferrocene
s, which were developed in the 1950s.
Often ansa-metallocenes are described in terms of the angle defined by the two Cp rings. In titanocene dichloride
, this angle is 58.5° whereas in the ansa-titanocene Me2Si(C5H4)2TiCl2 the angle is 51.2°.
The prototypical linker groups are of the type (CH2)n where n = 1, 2, and 3. More easily installed are linker groups consisting of heteroatoms, e.g. (CH3)2Si.
One potential application of ansa-metallocenes is in polymerisation reactions. Specifically, the ansa bridge allows greater control of the stereochemistry at the metal centre. For example, three distinct ansa-zirconocene dichloride catalysts used in the synthesis of polypropylene
in the presence of methylaluminoxane
(MAO) can be used to direct whether the product is syndiotactic, isotactic, or atactic.
Sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CnHn, substituted derivatives and heterocyclic derivatives...
ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s that are linked by a bridging group such that both cyclopentadienyl groups are bound to the same metal. The link prevents rotation of the cyclopentadienyl ligand and often modifies the structure and reactivity of the metal center. Some ansa-metallocenes are active in Ziegler-Natta catalysis, although none are used commercially. The term ansa-metallocene (ansa being Greek for "handle") was coined by Lüttringhaus and Kullick to describe alkylidene-bridged ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
s, which were developed in the 1950s.
Often ansa-metallocenes are described in terms of the angle defined by the two Cp rings. In titanocene dichloride
Titanocene dichloride
Titanocene dichloride is the organotitanium compound with the formula 2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air...
, this angle is 58.5° whereas in the ansa-titanocene Me2Si(C5H4)2TiCl2 the angle is 51.2°.
The prototypical linker groups are of the type (CH2)n where n = 1, 2, and 3. More easily installed are linker groups consisting of heteroatoms, e.g. (CH3)2Si.
One potential application of ansa-metallocenes is in polymerisation reactions. Specifically, the ansa bridge allows greater control of the stereochemistry at the metal centre. For example, three distinct ansa-zirconocene dichloride catalysts used in the synthesis of polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
in the presence of methylaluminoxane
Methylaluminoxane
Methylaluminoxane, commonly called MAO, is a white solid with the general formula n.-Physical properties:MAO is pyrophoric, and is violently reactive with any chemical bearing an acidic proton. However, MAO is generally used as a solution in an hydrocarbon due to its relatively high solubility in...
(MAO) can be used to direct whether the product is syndiotactic, isotactic, or atactic.